This disclosure relates generally to optical measurement of integrated circuit performance, and in particular but not exclusively, relates to measurement of statistical variation of electrical signal phase in integrated circuits (such as for signal jitter) using a time-correlated photon counting system.
Microprocessor designers are continuously trying to design faster and more powerful microprocessors. The speed of a microprocessor can at times, however, be limited based on various factors present in the overall system. One of these factors is a periodic electrical clock signal. The microprocessor performs its operations, timing, and synchronization based on the edges of the clock signal. Due to inherent deficiencies in the clock signal, microprocessors typically have to be designed to account for such deficiencies and therefore may not be able to operate as optimally as desired.
As an illustration, a deficiency in the clock signal is signal jitter, which is a perturbation in phase. This jitter is typically caused by random voltage fluctuations in nodes of the circuit but can also be influenced by factors such as the type of circuit, semiconductor material used, operating conditions, and so on. Because of this jitter, the edges of the clock signal do not always exactly occur at the same time for all cycles and instead vary randomly from one clock edge to another.
Due to this jitter, microprocessors are designed to operate within xe2x80x9cguard bandxe2x80x9d error margins. That is, the operations of the microprocessor that are dependent on the clock edges are delayed until it is certain that the fluctuating edges of the clock signal have occurred sometime during the guard bands, and then the microprocessor proceeds with its operations after the guard bands have passed. Implementation of longer guard bands ensures that clock edges have occurred, but undesirably causes the microprocessor to lose operating speed since it must delay its operations until after the guard bands have passed. In contrast, implementation of shorter guard bands does improve speed of the microprocessor, but operational errors may occur if clock edges fall outside of the guard bands.
These problems are multiplied if there are multiple clocks on a chip and/or if the clock(s) branch out to a tree. If there is error at one part of the chip due to clock jitter, then the error and the clock jitter can propagate to the other circuit parts of the chip and cause synchronization and timing problems.
In an effort to measure circuit performance to determine the jitter characteristics, several techniques are used. One technique is to use mechanical pico probes. The mechanical probe is placed on a node, and the signal is measured in an analog manner in an oscilloscope (by watching the clock edges). However, the mechanical probe itself introduces parasitic capacitance onto the node, and therefore, this is invasive because the mechanical probe disturbs the node by altering the circuit and contributes error to the measurement. Moreover, mechanical probing produces unsatisfactory results at high frequencies.
Another technique to measure the jitter performance is to use laser-voltage probing. With laser-voltage probing, a pulsed laser is directed at points on the device under test (DUT), such as at transistor junctions, where the index of refraction changes when the transistor switches. The reflected light can then be viewed and used as a basis for measuring the jitter. However, it is very difficult to obtain jitter measurements with this technique, and the laser itself is invasive to the DUT since it somewhat interacts with the DUT and can thus contribute error.